ece124: add clocks

docs/1b/ece124.md

@@ -308,6 +308,30 @@ A **demultiplexer** has one input, $n$ select inputs, and up to $2^n$ outputs th
 
 A **binary encoder** takes $2^n$ inputs and $n$ outputs, with the binary representation of the $n$ outputs indicating the inputs enabled by binary index.
 
+## Sequential circuits
+
+!!! definition
+    - A **combinatorial circuit** is dependent on present signals.
+    - A **sequential circuit** is dependent on past and present signals, using storage elements to track state.
+
+**Synchronous** sequential circuits only change signals at discrete times, such as with clock signals. Asynchronous circuits change whenever.
+
+### Clocks
+
+!!! definition
+    - The **period** $t$ is the duration of one clock cycle.
+    - The **frequency** $f$ is the reciprocal of the period.
+    - The **rising edge** is the instant a clock changes from $0$ to $1$.
+    - The **falling edge** is the instant a clock changes from $1$ to $0$.
+
+### Storage elements
+
+A **basic latch** changes based on its input signal level such that it is level-sensitive.
+
+A **gated latch** is a basic latch as well as a control input that locks the current state. The latch is only togglable when the control input is on.
+
+A **flip-flop** contains two gated latches and a control input. The state is only adjustable during the edges of the control signal, so it can only change up to once per cycle.
+
 ## VHDL
 
 VHDL is a hardware schematic language.